Moving-coil electrical instrument



May 5, 1953 D. A. YOUNG MOVING-COIL ELECTRICAL INSTRUMENT 3 Shee't sheat l Filed Jan. 3 1948 INVENTOR m m M J b w p D. A. YOUNG MOVING-COIL ELECTRICAL INSTRUMENT May 5, 1953 3 Sheets-Sheet 2 Filed Jan. 30, 1948 ATTORNEY May 5, 1953 D. A. YOUNG MOVING-COIL ELECTRICAL INSTRUMENT 3 Sheets-Sheet 3 Filed Jar 30, 1948 INVENTOR .Douy/QJJA )aung.

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ATTORNEY Patented May 5, 1953 UNITED STATES PATENT OFFICE MOVING-COIL ELECTRICAL INSTRUMENT Douglass A. Young, East Orange, N. J assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 30, 1948, Serial No. 5,404

28 Claims. 1

This invention relates to moving-coil electrical instruments and it has particular relation to moving-coil electrical measuring instruments having ferromagnetic structures defining air gaps for the associated moving coils.

Moving coil instruments are employed in various fields, such as for measuring and relaying. The advantages of ferromagnetic structures for moving-coil instruments, particularly those of the electrodynamometer type, long have been recognized. The deterrents to adoption of such magnetic structures have been lack of accessibility and lack of accuracy. Ferromagnetic structures are structures constructed from a material, usually containing iron, which has a magnetic permeability substantially greater than that of a vacuum.

Recently moving-coil instruments of the electroclynamometer type have become available which include ferromagnetic structures defining air gaps for the associated moving coils, wherein accuracy is obtained without sacrifice of accessibility. Examples of such instruments will be found in the Young et a1. application, Serial No. 500,896, filed September 2, 1943, now Patent 2,438,027, and the Lunas application, Serial No. 570,028, filed December 27, 1944, now Patent 2,508,410.

In the aforesaid patent applications, a movingcoil is associated with a ferromagnetic structure which is divided into two spaced sections. These sections have passages permitting removal of the associated moving coil without disturbing the magnetic structure in any way. Each of the magnetic sections produces a solenoid force acting on the moving coil in response to energization of the moving coil alone, but the solenoid forces of the two sections are oppositely directed. Consequently, errors due to the solenoid forces are to a substantial extent eliminated.

Instruments of the type discussed above also appear to be subject to a force responsive to energization of the moving coil which urges the moving coil towards a position intermediate the ends of the path of travel of the coil, usually a midscale position. This force is smaller than the previously-mentioned solenoid forces, 'and for some applications of instruments, may be neglected. However, it does introduce an error and the magnitude of the error is dependent on the energization of the moving coil. This force appears to be due to magnetic flux produced by the moving coil, which circulates around a side of the moving coil located in the air gap of the associated magnetic structure, crossing and rein as a force F.

In accordance with the invention, a moving-coil instrument having a magnetic structure defining an air gap for the moving coil is designed to develop an auxiliary force or torque which compensates for the aforesaid force F. In a preferred embodiment of the invention, if two opposed solenoid torques are available as in the instruments previously disclosed in the aforesaid patent applications, these opposed torques are proportioned to compensate for the force F. Alternatively an auxiliary mechanism may be provided for developing the desired compensating force.

The invention further contemplates an im provement in scale distribution of a moving coil instrument having a laminated magnetic structure defining an air gap for the coil, wherein laminations are bent for th purpose of changing scale distribution.

It is, therefore, an object of the invention to provide an accurate moving-coil instrument having a ferro-magnetic structure defining one or more air gaps for the moving .coil.

It is a further object of the invention to provide a moving-coil instrument having a magnetic structure developing torques in response to the energization of the moving coil, urging the moving coil towards an intermediate position in its path of travel with means for compensating for such torques.

It is a still further object of the invention to provide an improved method for controlling the scale distribution of a measuring instrument employing a laminated magnetic structure.

Other objects of the invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is an exploded View in perspective with parts broken away of an electrical measuring instrument embodying the invention;

Fig. 2 is a view in perspective with parts broken away showing the instrument of Fig. l in assembled condition;

Fig. 3 is a view in top plan with parts broken away of the instrument shown in Figs. 1 and 2;

Fig. 4 is a view in side elevation of a two-element electrical measuring instrument;

Fig. 5 is a graphical representation of the solenoid forces or torques developed by the instrument of Fig. 1;

Fig. 5a is a view in side elevation with portions broken away of a modification of the instrument showninFigslandZ;

Fig. 10 is a View in-plan with partsbrokenaway of a further embodiment of the invention;

Fig. 11 is a view in side elevation of the structure shown in Fig. 10;

Fig. 12 is a view in lan taken on the line XII2GI of Fig. 13 showing. a stillfurther .modification of the invention;

Fig. 13 is a view in section taken onthe line XIIIXIII of Fig. 12; and

Fig. 14-. is an exploded view in perspective with parts broken away showing a still further-embodiment of the invention.

Referring to the drawing, Fig. 1 shows an .elec trodynamometer instrument which includes a stator assembly I and a rotor assemblyia.

The rotor assembly comprises a shaft .3 which is mounted for rotation by suitable bearing screws l3 which are part of the-stator assembly. The shaft 3 supports acoil 3 having coil sides and '1 parallel to the shaft, but spaced radially therefrom. A pair of spiral, flexible, electroconductive strips l5, I? have their inner ends securedto insulating bushings I3, 2| which are carried by the shaft. The outer ends of the strips are connected tolugs 23, 25 for the purpose of establishing connections between the movable coil 3 and an external circuit through conductors 2T, 29. The terminals of. the coil3 are connected to the inner ends of the spiralstrips |5, i'i,.respectively. For dam-ping purposes, the shaft ,3 carrier an electroconductive disk which is mounted for rotation between the poles of a permanent magnet 3i. A control spring 3| has its inner end connected .to .the shaft 9, and its outer end connected to a lever 33 adjustably secured to the stator assembly. The control spring biases the rotor assembly towards a predetermined angular position. Indicating means such as an arm 39 is attached to the shaft. This arm.

carries a pen 4| across the surface of a chart 43. The chart may be advanced continuously relative to the pen 4| in a manner well understood in the art.

The stator assembly includes a magnetic structure 45 which establishes magnetic paths for the magnetic fluxes produced by currents flowing in the coil 3 and in fixed windings 4i and 49 which areassociated with the magnetic structure. The magnetic structure 45 includes a ma netic section 5| having a substantially continuous magnetic body or rim portion 53 which substantially surrounds the shaft 9 and the coil 3. A pair of pole pieces 55 and 57 (Fig. 2). project from opposite interior surfacesv of the, rim portion 53 to provide arcuate pole faces adjacent the paths oftravel of the coil sides 5 and.'l.. In addition. the magnetic section 5| has a pair of cantilever shaft 9 by a distance sufficient to permit passage of the movable coil 3 therebetween. Further more, the magnetic cores 59 and fil have arcuate surfaces spaced from the pole pieces 55 -and 5lto provide a pair of arcuate air-gaps within whichv the coil sides 5 and l are disposed for movement. It will be noted that the magnetic cores 58 and 5| provide a substantially cylindrical magnetic core which is attached on opposite sides to the rim portion 53 and which has the passage 53 extending therethrough. Since the passage 53 communicates with the air gaps in which the coil sides 5 "and I are positioned, the coil 3 may be rotated in-a counterclockwise direction (as viewed looking at the rotor assembly from the control spring end) to bring the coil into alignment with the passage 53. The coil then may be moved in a direction parallel to the shaft 9 through the passage G3.

The magnetic section 5| may be formed of any suitable soft magnetic material having a magnetic permeability substantially greater than that of a vacuum. Such materials are termed ferromagnetic or generally magnetic materials, an example being silicon iron. Preferably, a materialhaving low hysteresis loss is employed. The magnetic section 5| may be formed of a solid piece of soft iron. However, it is preferable to form the magnetic section 5| of a plurality of laminations L as shown in Figs. 1 and 2, particu larly if the instrument is designed for measuring alternating-current quantities. The laminations may be provided with suitable openings 65 through which rivets may be passed for the purpose of securing the laminations together. If the magnetic section is formed of laminations, the

desired contour of each lamination may be ac curately formed by a punching operation.

By inspection of Figs. 1 and 2,,it will be .noted that a separate magnetic path is providedfor each of the coil sides 5 and I. The magnetic path for the coil side 5 includes the pole piece 55 and the magnetic core 59 together with. the air-gap therebetween. The winding 41, when energized, directs magnetic flux through this magnetic path to provide a magnetic field for the coil side 5. In a similar manner, the magnetic path for the coil side 1 includes the magnetic core 6| and the pole piece 51 together with theair gap therebetween, When the winding 49 is energized, magnetic flux is directed through the associated magnetic path to. establish a magnetic field for the coil side 1.

Although the magnetic section 5| alone may be employed, an improvement in performance may be obtained by adding thereto an additional magnetic section 61. The reason for this additional magnetic section may be understood by consideringthe solenoidaction of the magnetic section 5| when employed alone. The magnetic section 5| is asymmetric with respect to the path of travel of the movable coil 3. When the coil 3 is in its extreme counterclockwise position (looking at the rotor assembly from the control spring end thereof) the. magnetic reluctance of the magnetic path offered to magnetic flux produced by current flowing through the coil 3 is a maximum. Conversely, when the coil 3 isadjacent its extreme clockwise position the magnetic reluctance offered to magnetic flux produced by currentfiowing in the coil 3 is a minimum. Consequently, when the coil 3 is energized and the windings 41 and .49 are deenergized, the coil 3 tends to take a position wherein the magnetic reluctance of the associated magnetic. pathis a minimum. This maybe termed a solenoid action and the force applied to the coil 3.by. the solenoid action urges the coil in a clockwise direction. In some. cases, as whenthe energization ,ofthe. fixed windings isconstant, .it is possible to calibrate the instrument to read correctly despite the presence of this solenoid action. However, this solenoid action is substantially compensated by the provision of the additional magnetic section 61. The compensation permits more correct indication by the instrument for substantially all applications thereof.

The magnetic section 6'! is similar in construction to the magnetic section but is reversed with respect to the magnetic section 5| about a line transverse to the shaft 9. The magnetic section 91 has a pair of magnetic cores 69 and H which extend through the coil 3 on opposite sides of the shaft 9. In addition, the magnetic section El has a pair of pole pieces 13 and 15 which are positioned respectively in the windings 41 and 49. It will be observed that the magnetic cores E9 and 1| are spaced to provide a passage 11 therebetween which corresponds to the passage 63 of the magnetic section 5|.

Since the magnetic section 31 is reversed with respect to the magnetic section 5|, the force due to solenoid action which is applied thereby to the coil 3 is opposed to the force developed by the solenoid action of the magnetic section 5|. Consequently, the resultant magnetic structure is substantially free of errors resulting from such solenoid action. The magnetic cores 59, 6|, 69 and H all pass through the coil 3 to form a resultant magnetic core therefor which is substantially symmetric with respect to the path of movement of the coil 3.

By inspection of Fig. 1, it will be observed that the passages 53 and 11 are displaced angularly about the shaft 9 with respect to each other. Consequently, the coil 3 cannot be removed from the magnetic structure 45 by a simple movement thereof in the direction of the shaft 9. To permit removal of the coil from the magnetic structure the magnetic sections 5| and 61 are spaced from each other along the shaft 9 by a distance sufficient to permit movement of a side of the coil 3 therebetween. The desired spacing may be provided by any suitable spacer formed of either magnetic or nonmagnetic material. In the embodiment illustrated in Figs. 1 and 2, the spacer is divided into two parts 19 and 1911.. Each of the parts is in the form of a plurality of magnetic laminations which are similar in construction to the adjacent parts of the laminations of the magnetic section 5 In order to facilitate inspection of the space between the magnetic sections 5| and 51 after assembly thereof, the parts 19 and 19a are located at a substantial distance from each other to provide an opening 19b (Fig. 2) in the magnetic structure 45. The space between the magnetic sections is clearly visible through this opening. Consequently, when the magnetic sections 5| and 61 are assembled with the spacer therebetween as shown in Fig. 2, a space is provided between the pair of magnetic cores 59 and 6| and the pair of cores 69 and 1|. This space is sufficient to permit movement of a side of the coil 3 therebetween.

It is believed that the operations required to assemble and disassemble the instrument illustrated in 1 and 2 are apparent from the foregoing discussion. To facilitate a further description of such operations, reference will be made to a leading side 9a of the coil 3 (the lower side of the coil 9 as viewed in Figs. 1 and 2), and a trailing side 36' (the upper side of the coil as viewed in Figs. 1 and 2). It will be understood thatthe magnetic structure 45 comprising the laminations of the magnetic. sections 5| and 61 and the laminations of the spacer is first completely assembled as shown in Fig. 2 wherein a rivet 45a is disclosed for uniting the laminations to each other. Also the rotor assembly la is completely assembled, the complete assembly including the shaft 9, the coil 3, the conductor strips 5 and I1, the disk 35. the arm 39 and the control spring 3|. The rotor assembly then is placed above the magnetic structure 45 (as viewed in Figs. 1 and 2) with the leading side 3a of the coil aligned with the passage 63 of the magnetic section 5|. The rotor assembly including the coil 3 is then lowered in a direction parallel to the axis 9 to pass the leading side 3a completely through the passage 93. The leading side 3a of the coil now is positioned between the pair of magnetic cores 59 and 6| and the pair of cores 69 and H.

To complete the insertion of the coil 3 into operative position, the rotor assembly including the coil next is rotated in a clockwise direction (looking at the rotor assembly from the controlspring end thereof) to bring the leading side 3a of the coil into alignment with the passage 11 of the magnetic section 61. During such rotation of the coil 3 the leading side 3a moves between the magnetic sections 5| and 61. After the leading side 3a has been brought into alignment with the passage 11, the rotor assembly is lowered in a direction parallel to the shaft 9 to pass the leading side 3a completely through the passage 11. The coil now is positioned to embrace the complete resultant magnetic core formed by the magnetic cores 59, 6|, B9 and H. The bearing screw II and the support therefor are next placed in position, and the bearing screws II and I3 are adjusted to mount the rotor assembly for rotation with respect to the magnetic structure. The outer ends of the conductor strips [5 and H are soldered to the lugs 23 and 25 and the permanent magnet 31 is positioned as shown in Fig. 1. To complete the installation of the rotor assembly, the outer end of the control spring 3| is soldered or otherwise secured to the lever 33. By following a reverse procedure the rotor assembly Ia may be removed from the magnetic structure 45 without disturbing the magnetic structure in any way.

From the foregoing discussion, it is clear that the magnetic structure 45 is formed of a plurality of unitary laminations each of which has integral pole pieces and magnetic cores. Because of this construction the magnetic structure may be provided with accurate air gaps, and the accuracy of the air gaps is not disturbed by assembly or disassembly of the instrument.

In certain applications a two-element electrodynamic instrument is required. Such an instrument may be constructed in the manner illustrated in Fig. 4. Referring to Fig. 4, a two-element electrodynamic instrument is disclosed which includes the two elements SI and 83. The element 8| comprises a magnetic structure 95 which is similar in construction to the magnetic structure 45 of Figs. 1 and 2. It will be observed that the magnetic structure has associated therewith a pair of fixed windings 8! and 89 which correspond to the fixed windings 41 and 49 of Figs. 1 and 2. In addition, the magnetic structure 85 has disposed therein a movable coil 9| which corresponds to the movable coil 3 of Figs. 1 and 2. The element 83 is similar in construction to the element 8| and includes a magnetic structure 93, fixed windings and 97 and a movable coil 99. The magnetic structures 85 and.

enema:

7 "were mounted; on;sui tab1esupporting posts It andare :spaced-fromeach other 'sufliciently to permitirotationvof one of the movable-coils therebetween. In'certaincases: it may be desirable to place s'magneticnshields 102: between the fixed windings-81- and 95 and-between the fixed windings .89 :and .91 to prevent imagnetic interference between theiwindings oncopposite sides of the shields;

Thermovztble coilsrsi and =99 are secured toe. common-shaft 103 ion rotation therewith. This shaft: carries farpair -of.:conductor strips 1 l 05 for connectingzthe terminals of. the movable coil 99 to ran externalcircuit andza pair of'conductor strips 2: l 07 for :connecting the 1 terminals of the movablecoil- ,9i rto=anexternalcircuit. These conductor strips correspond to' the conductor strips 15 and ll of Fig. 1. In addition, the disk 35,;the; arm 38-iwhichrmay-support anindicating pointer'or arecordingpen) ,and the control spring 3|: are secured: to the shaft 103 with the disk 35 positioned iorunovement: between thelpolesiof the permanent magnet 3'1.- As well understood in the art,i'each:of the :elements Si and 33 may be energized'vfromgaseparate pairof conductors of a three-.wire circuit or froi'n 'a separate phase of a polyphase circuit.-

, Since the principles-employed in the constructionoffithe instrument illustrated in Figs. 1 and-2 arealso-employedifor the instrument of Fig. it

followsthat the rotor; assembly Fig. v4 may be introduced: into :opcrativeposition with respect to the-magnetic structurest 5 and i 93 or may be removed therefrom without disturbing the -mag-. neticw structures .in anyway. For example, in constructing the instrument, the magnetic structures.85 Land :93 tarecompletedand are secured to the'supporting posts lfli. Forconvenience in discussingszthe assembly of .theuinstrument, the coil 199 'will be referred to as having :a leadingside 9.9a anda trailingiside'fiflb. The coil 8i will be referred'toas having a leadingside 98a and a trailingside 9th. This corresponds to the notation employed forthe coil 3 of. Figs. 1 and 2. The rotor assembly of. 'Fig.:4 is first completely assembled. This rotor assembly includes the shaft i'3,=the coilsBl 'and-Qfipthe conductor strips m and WT, the disk 35, the pen arm 39 and the controlspring 3 I .The rotor assembly then ispiaced above the magnetic structurett as viewed in .Fig.

4 with the leading side. 99a of the coil 99 positioneduabove the adjacent passage in'the magnetic structure :85. The rotor assembly .then is dropped in a directionzparallel to the shaft I83 rotated and again dropped to-position the coil .99 for embracingthe magnetic cores of the magnetic istru'cture185. This procedure is :exactly similarptosthat employed'ior dropping the coil-3 of Figs. 1 and 2 through the passages '53 and :"i toqembrace the associated magnetic cores.

The coil 99 then is passed completely through the magnetic structure "85 byrotating the coil until'its trailing-side 69b is in'position to drop through the adjacent passagein the. magnetic structure 85.: After the trailing side has passed through the adjacent passage, the coil 99 is retated to. pass ,thetrailing side 9% between the magnetic sections of 'the. magneticstructure S5 untilthe trailingside- Qiib-ispositioned to drop through the-lower passage in the magnetic structure." The coil QQinoW is lowered to a position between-the. magnetic structures 85- and 93.

The operation of passing-a coil completely through. its: magnetic structure may be understood more 'fully by- ;a further consideration of- Figs. 1 and .2; Assuming that the coil. 3 isintheposition illustratedaimFigsl, 1 and 2and that'it is desired to dropthe coil-completely through its associated structure 45,. the-coilis rotated until its trailingside .3b is adjacent the passage 63 in the magnetic section 5 l. The coil now is lowered until the trailing .-.side 131) -is positoned between themagnetc sections 5| and'61. By suitablyrotatingthe 'coil:3 ina clockwise direction (looking at the rotor assembly from/the control spring end thereof-l the trailingside 3b is moved through the magnetic sections 5 and 6l to a position wherein the-trailing side-is inlalignmentwith the passage H in thewmagneticisection 61. The trailing side 3b nowmay be dropped" through the passage 11 to complete the passage'of the coil 3 through its associated magnetic structure -45: The operation of passing the ooil=99 0f. Fig.4 completely through the magnetic structure is similan to that discussed for the coil 3 of-Figsl and 2. I

With theceil 99 positioned-between the magnetic structures :85" and 93 .-of: Fig; 3, the rotor-asscmbly .is rotated to bring the leading side99a of the coil=99 into-alignment with. the adjacent passage of the magnetic: structure 93.- The 'coil 99 next is dropped, rotated andagain dropped in the manner:previous1y,.-discussed with reference tothe coilv 3 or" Figs. 1 'and'2 until the coil 99 is in position to embrace the magnetic cores of the associated magnetic structure .93. Since the mag! netic structures- 85 and'93 are similar, the movement of the roil 99 from a position between the magnetic structures 85 and 93 to a position wherein the coil 99' embracesthe magnetic coresof the magnetic structure 93 also moves the coil! froma position above the magnetic structure 85 to a position wherein the coil. 9i embraces the magetic cores of the-associated mzugneti'c structure 85. -Consequentlmrboth of'the coils 9| and99- are in. their operative positions with respect to theirassociatedmagneticstructures. With the rotor assembly .of 'Fig. 4 :in this position, the bearings associated'with the shaft I 83 may beadjusted and the outer ends of the conductor strips I05 and I6"! maybe connected'asdiscussed' with referenceto Figs. 1 and 2; Injaddition, theouter end ofthe control spring 3 I' may be connected to "its associated :lever. 33' and' the ::pcrmanent magnet 37 may be moved'tc operative position 'Withrespect to the disk:35. The instrument of 'Fig.':,4'now is in. completely "assembled. condition. By following a reverse-precedure, the rotor' assembly .of Fig. '4

may be removed: completely from thefmagneticstructures 85v and'93' without'di'sturbing the'mag-J netic' structures inianyway. 1'

Referring again. to Figs.;:1 ahdZ, the windings 61 and 69. are connected in series-and are so energized that if direct current -is passed ..therethrough, magnetic =iiux "flows through the pole pieces inthe directions illustrated'by' the arrows would be exactly .similar-to instrumentsLillus tr'ated and describedintheaforesaid Lunas'patent application. The reversely related sections and 61 eliminate to a substantial extent the solenoid forces resulting from energization of the moving coil alone. However, a force exists which tends to move the moving coil towards a midscale position. It will be helpful to review briefly the present understanding of the theory underlying the force acting on the moving coil.

Referring to Fig. 3, it will be noted that when the windings s1 and 49 are deenergized and the moving coil 3 is energized, magnetic flux is produced which passes through the coil 3 and crosses the air gap between the magnetic core 59 and the pole piece 55. This magnetic fiux is illustrated in Fig. 3 by dotted lines It will be understood that magnetic flux crosses each of the air gaps of the sections in substantially the same manner.

The magnetic flux represented by the dotted lines Ii! produces a solenoid force which urges the moving coil 3 in a clockwise direction as viewed in Fig. 3. However, since the solenoid forces developed by the two magnetic sections 5! and 5'! are in opposition, such solenoid forces substantially cancel.

Current passing through the moving coil 3 also produces magnetic flux which follows a path represented in Fig 3 by dotted lines H3. It will be noted that this path extends from the magnetic core "I! to the pole piece 15 acros the air gap therebetween and recrosses the air gap to return to the magnetic core. This magnetic flux urges the moving coil 3 towards a position wherein the magnetic reluctance offered to the magnetic flux is a minimum. For the structure of Fig. 3, the moving coil 3 is urged towards a midscale position. Since the magnetic flux represented by the dotted lines H3 crosses the air gap twice, the magnetic reluctance of the path through which the magnetic flux flows is large compared to the magnetic reluctance of the path followed by the magnetic flux represented by the dotted lines I I I. As previously pointed out, the force developed by the magnetic flux represented by the dotted lines i3 will be termed a force F. For some applications, the error introduced by the force F may be neglected. However, it is desirable that this source of error be eliminated insofar as possible. It will be understood that magnetic flux similar to that represented by the dotted lines.

i3 is present in each of the air gaps of the magnetic sections 5i and t'l.

The problem may be considered further with reference to Fig. 5, wherein ordinates represent torque applied to the moving coil 3 of Fig. 3 and:

abscissae represent the angle of displacement of the moving coil 3 from its zero position which is assumed to be the counterclockwise end of the path of travel of the moving coil in Fig. 3. The

force F developed by a predetermined current:

in the midscale position of the coil 3 (45), no

force F is applied to the moving coil.

If all laminations of the magnetic sections 5! and 57 are similar to the previously described laminations L, the previously mentioned solenoid forces developed by the magnetic sections 5! and 61, when the coil 3 is energized by the predetermined current, may be assumedto be represented in Fig. 5 by curves H5 and H6. These solenoid forces are assumed to be uniform, oppositely directed and equal in magnitude throughout the angular movement of the coil. Consequently, these solenoid forces fully compensate each other. If the forces represented by the curves Fl, H5 and H5 alone act on the moving coil 3, the moving coil seeks a midscale position (45 in Fig. 5).

If the solenoid forces are deliberately unbalanced. the coil may be made to seek other scale positions. For example, let it be assumed that the magnetic section 5! has laminations removed and the magnetic section 6'! has magnetic sections added to produce solenoid forces represented by the dotted curves Il5a'and lifia of Fig. 5. fhese curves have a constant difference throughout the angular path of the coil 3 which is represented by the dotted curve I ll.

The force represented by the curve I ll! is equal in magnitude and opposite in direction to the force represented by the curve F! only when the coil 3 occupies its zero or extreme counterclockwise position. Consequently, when the coil 3 is in its zero position, it would be free of any error resulting from the application of the three forces represented by the curves Fl, !5a and I lea. However, when the coil 3 isadjacent its extreme clockwise position in Fig. 5), the error introduced by the three forces would be larger as a result of the unbalance represented by the curves 511 and llta. By selection of the numbers of laminations in the magnetic sections 5! and 6'! to provide the required amount and direction of unbalance represented by the curve H, the force F! may be compensated at any one angular position of the moving coil 3. Forexample, the compensated position may correspond to the position of the moving coil when the coil occupies its zero position, whether the instrument is a left-zero, a right-zero or a center-zero instrument.

A structure providing an unbalance force similar to that represented by the curve H! is illustrated inFig. 5a. The magnetic sections A5! and A6! of Fig. 5a correspond respectively to the sections 5 and 6'! ofFigs. 1 and 2, and are constructed entirely of the laminations L. Otherwise, the instrument is similar to that illustrated in Figs. 1 and 2. By inspection of Fig. 5a it will be noted that the magnetic section A51 has a larger number of laminations than the number employed for. the magnetic section A5! to produce the unbalance force represented by the curve ll'l. If the magnetic section A5! were to have a larger number of laminations than the the magnetic sections or in the numbers of laminations employed for the magnetic sections.

.ln'order to compensate for the force F, a compensating force or torque represented by the curve I8 is required. The force represented by the curve H8 may be obtained by suitable shaping-of the magnetic cores 59, 6!, 69 and H.

Conveniently, such shaping maybe obtained by suitable punching of one or more of the laminations L. In the embodiment of Figs. 1, 2 and 3, certain of the laminations La are modified to produce the desired force. By inspection of the magnetic section 5! of Fig. 1, it will be observed that each of the laminations La is similar to each of the 'laminations L, except for. the-shaping; of v the magnetic cores. Th lanimations La provide magnetic "cores 69a and 1 ta which 1 are tapered somewhat more than the :magneticcores 69' and H of'thelaminations L. It will be understood that the magnetic-section! bisexac'tly similar to p the :magneticsection? 6l,--except-*forthe reversal thereof about a-line transverse to the shaft 9.

The effect of the 'tapering "-of the== magnetic cores is tovary 'thesolenoid' force developed by each of the -magnetic 'sections-*5l "and' fil as a 'sented-by" the curve H8. "Consequently-by suitable shaping of theflmagneticscores;awcompem sating force represented'by -thei curve [IS-of- Fig. 5" may' beobtainejd, which-substantially compensates for the force represented" byfithe' curve F l.

The: forces" represented: in'---Fig;ri5-iare for a predetermined current flowing through the coil 3 Inasmuch as all of 1 these forces vary'in--magnitude in accordance with variations irrmagnitude 'of the coil current; the-compensation'is effective for'all ener glza'ti'ons [of the coil.

Although the-outer ulamina-tions (those adj acent the coil:sidestwandtb'lmf the'secticns 5i and 61 may be tapered," the-location of-' the tapered laminationsIiwadjacent the coil sides- 3a and 3b may make'the-operationof the instruments' sensitive "to" exact axial positioning; of the coil 3. For thisrea'son;the-laminations La preferably. are placed-interiorly ofthe resultant magnetic structure; such as adjacent=thespacers l9 and'lSa.

Referring" to ,Figs: I and '3, 'it -willi be observed that the magnetic corestflai and H a-provide air gaps whichincrease irr'l'en'gth'adjacent the coil 3- as the coil moves'tin" a: clockwise direction; The

tapering ofthemagnetic-cores",associated with H the laminations L'cvdoes -not appreciably affect.- the resultant 1 efiective distributionr of magnetic flux in the" air. gaps" as a resultofenergization of the windingsl'hand fl: ThiSffOllOWSlI'OlIl the fact that the "magnetic sections are reversed with respect to'each o'ther. Since the magnetic cores associated with 'the magnetic" sections 5| and 6'! taper "in: opposite directions; the resultant efi'ective air" gap length remains substantially constant throughout the path'ofi travel of the moving coil.

The :instrument" of FigrG employs" a magnetic structure 45c" whichis; similar to? the magnetic structure 45 of- Fig. 1, except that-"all" of the laminations in the structure' 45c' are substantially the same as thejlaminations L'of -Fig: 1. pensation' for the force? is obtained in Fig. 6 by a magnetic strip l25 'whichmay "be-constructed of iron. Thisstripis securedto a magne'ticcore 6 l c which corresponds -tof-the= core-.- 6 l.-. of: Fig. l.

The magnetic strip I25 has: its ends; 1-2 511:". and

l25b bent to form a pocketateach end :of: the strip. The instrument cf iFigz- 6: also remploys. a moving coil" 30 which correspondsrto: the.:mov-

ing coil 3*of-Fig: 1. (By inspection-of Eigsz'fiand 7, itwill be-observedthat the uppertcoitside. 3cb

of the-coil 3c ispositionedfommovement through the pockets" formed "bythe magneticmstnpa I -25. When the coil side-*3 'cb is disposed-i 11120116 of,..the

, pockets, such as that fonned's': by .thez end: 1 25a,

a path is establlshedtformagnetic flux produced;

Cornbe :employed as desired. "magnetic strip l29= is secured to the magnetic core 590 which corresponds to the magneticcore A 59-of Fig. 1. -actlysimilarin construction to the strip I25.

by-currentnowing through the coil. This path is indicated in-Fig. 7 by a dotted line I21. "This rmagnetic fluxproduces a solenoid force "which --tends tourge the coil side 3017 farther into the pocket. Similarly; when the coil side 3cb: is'located in the Docket'formedby the end l25bof the magnetic strip; current flowing through the moving coilproduces I a force urging the moving c011 farther into the last-named pocket. Consequently,- by properlyshaping the ends la: and I256; the forceexerted thereby on themoving' coil "30-" may be proportioned to compensate for the iforce F-developed by current flowing through the coil.

Gne -or more of themagnetic strips l25-may In Fig. 6, a second The magnetic strip I28 may be ex- The ends I25a and I251) are spaced by a'distance'sufilcient'to permit passageof the 'movin associated magnetic cores. ture ifid has magnetic cores59d and Bid which -corresponcl' to the magnetic cores 59c and tile of =-"Fig.-6. "However, the inner faces of these ma coil therebetween. The moving coil has alength sufficient to permit it to be raised sufficiently to clear the ends of the magnetic strip. This per- --mits removal of themoving coil from the as sociated magnetic structure inthe manner discussed with reference to- Figs; 1 and 2.

"In the embodiment-of Fig.-- 8,amagnetic structure d'is employed which is similar to the masnetic structure 450, except for the shaping-of the The magnetic strucnetic coreshave noncylindrical configurations to provide pole faces l3! and I33, respectively.

InFig. 8; theslngle coil-3 of Fig. 1 is replaced by two coils 135 and 137 which may be connected in series to serve in' place-0f thecoil 3. 1 135 links the magnetiecoresSSd and 69d which gorrespond tothe magnetic'cores 59 and 69 of lg. l.

The coil Thecoil I31 links the magnetic cores Bid and *Hd which correspond to the'cores BI and" of'Fig. 1. The provision of the doublecoil-structure' illustrated in Fig. 8' is discussed in" the aforesaid Lunas patent application.

? torques to the associated shaft 9.

.coils I35 and I31. are energized; magnetic-flux is produced which follows-paths represented by the dottedlines HI and [43in Fig. 8.

If a magnetically, astatic instrument isdesired,

' the windings 41 and Mmay be connected to direct magnetic'flux respectively in accordance with the arrowslflila and. llfla. For alternating current energizatiom the arrows represent instantaneous directions. The coils I35 and 13'! may be energized with proper polarities toapply cumulative Theshaft 9 also has secured thereto amagnetic armature. I39 which has slotted ends for receiving respectively the inner coil sides of the Consequently, when the coils .The pole faces l3! and I33 are so shaped that I39 are av maximum when the Consequently, when the coil assembly 'isdisplaced from its midscale position, the armature 'I39-acts-.as the-armatureof an attraction mechanism to urgethe coil assembly away from the midscale; position. By proper h p ngofthe pole pieces 13! and-1'33 andby proper; dimensioning. 0f the. :armature. .l 39; the

13 forces developed by the armature may be proportioned to compensate for the resultant force F developed by current flowing through the coils I and I31.

Scale distribution may be controlled to a substantial extent by bending one or more of the laminations employed in the various magnetic structures. For example, in Fig. 8, the magnetic structure d includes a pole piece 5512 which corresponds to the pole piece of Fig. 1. If the sensitivity of the instrument is too great when the coil assembly is adjacent its extreme clockwise position as viewed in Fig. 8, the sensitivity may be decreased by bending a portion I45 of one or more of the laminations adjacent the right-hand end of the pole piece 55d. Consequently, by bending any portion of the laminations away from their effective positions, the sensitivity of the instrument may be decreased a desired.

Fig. 10 shows a magnetic structure 456 which is similar to the magnetic structure 45c of Fig. 6. The rotor assembly associated with the ma netic structure 45c may be similar to that illustrated in Fig. 1, except for the addition of a magnetic armature I41. This magnetic armature rotates between the pole faces M9 and I of an electromagnet I53 which has an energizing winding 155 connected in series with the coil 3. The pole faces I49 and I5l are shaped to provide air gaps between the ends of the armature I41 and the pole faces which are a maximum when the coil 3 is in its midscale position and a minimum when the coil 3 is at either of its end positions. Consequently, the armature I41 and the electromagnet I53 comprise an attraction mechanism which operates in substantially the same manner as the armature I39 and associated parts of Fig. 8. By proper shaping of the pole faces I49 and l5l, by proper dimensioning of the armature I41, and by proper energization of the electromagnet I53, the force developed by the armature 141 may be proportioned to compensate for the force F resulting from current flowing in the coil 3.

In Fig. 1, certain of the laminations La had their magnetic cores tapered to increase the lengths of the associated air gaps. The tapering of the magnetic cores also may be effected by cutting off the tips of certain of the laminations as illustrated in Figs. 12 and 13. Although the number of laminations to be cut may vary, it will be assumed that four laminations, ILa, ZLa, 3La and 4La are to be cut in each of the sections 5lf and 61f. These sections correspond to the sections 51 and 61 of Fig. 1. The laminations lLa, ZLa, 3La and 4La correspond to the laminations La of Fig. 1. By inspection of Fig. 12, it will be observed that the laminations ILa, ZLa, 3La and 4La have their core ends cut off by increasing increments. The efieot of such cuttin is to taper the resulting magnetic core and to taper the effective width of the air gap. Since the instrument of Figs. 12 and 13 operates in substantially the same manner described with reference to Figs. 1, 2 and 3, further discussion thereof is believed to be unnecessary.

Any of the foregoing embodiments may be incorporated in a circular scale instrument wherein a moving coil has only one side disposed in the air gap of an associated magnetic structure. As a specific example, a circular scale instrument is illustrated in Fig. 14 and is provided with compensation similar in substance to that discussed with reference to Figs. 6 and 7.

In Fig. 14, a moving coil C3 is secured to the shaft 9. Except for the coil C3, the rotor assembly may be similar to that illustrated in Fig. 1, and mounts the coil C3 for rotation with respect to a magnetic structure M. The magnetic structure M includes a magnetic portion A having an annular magnetic core Al. This annular magnetic core is proportioned to pass through the coil C3 and has a channel A3 extending radially from the interior to the exterior of the annular core for the purpose of permitting passage of a side of the coil C3 therethrough. The annular core Al has a magnetic member A5 projecting therefrom adjacent the channel A3 to connect the annular core Al to an outer magnetic element AT. The annular core Al and the magnetic element A! have adjacent surfaces which are spaced to define an annular air gap A9 within which a side of the coil C3 is positioned for rotation. This annular air gap may be of sumcient length to permit angular rotation of the coil C3 about the axis of the shaft 9 for an angular distance of the order of 250. It will be observed that the annular core AI and the magnetic member A5 are substantially in the form of a hook wherein the annular core Al is the hook section and the magnetic member A5 is the shank section. A fixed winding AB surrounds the magnetic member A5 and when energized produces a magnetic field in the annular air gap A9.

Because of the channel A3, the annular core Al for the coil C3 is asymmetric with respect to the path of travel of the coil. Such asymmetry is undesirable because of the solenoid action resulting from current flowing through the coil C3. This may be understood by assuming that the coil AB is deenergized and that a current flows in the coil C3. Under these conditions, no torque should be applied to the shaft 9 by the coil C3. However, because of the asymmetry of the annular magnetic core, the coil C3 tends to move to a position wherein the reluctance of the magnetic path associated therewith is a minimum.

In order to eliminate substantially this solenoid action, the magnetic structure M includes a second magnetic portion B which is similar to the magnetic portion A, but which is reversed with respect to the magnetic portion A about an axis perpendicular to the shaft 9 and parallel to the magnetic member A5. Since the magnetic portions A and B are similar in construction, parts of the magnetic portion B will be designated by the reference character B followed by the numeral applied to the corresponding part of the magnetic portion A.

By inspection of Fig. 14, it will be observed that the asymmetries of the magnetic portions A and B with respect to the path of travel of the coil C3 are such as to produce a resultant magnetic structure which is substantially symmetric with respect to the path of travel of the coil.

This is accomplished by positioning the channels A3 and B3 adjacent opposite ends of the path of travel. of the coil 93. As a result of this construction, torque resulting from solenoid action is to a substantial extent eliminated.

In order to permit the insertion of a preformed coil into embracing relationship with the annular magnetic cores Al and Bi, the magnetic portions A and B are spaced axially along the shaft 9 in any suitable manner fora distance sufficient to permit passage of a side of the coil C3 therebetween. The 'spaceris a magnetic or non-magnetic structure S which, if desired, may be simimosmoi .1. I lar:. to .the .magnetic aportionyzArrexceptii for? the omission nor the annular magnetic :acOIe Al- 'Although the magnetic. portionsA and B and :the

spacer S may be formed of magneticallwsoftiron or steel ofsolidpsection, rp-reierably they are laminated, as illustrated in Fig. 114.

With the construction of the: circular;:sc,ale instrument. as thus far described. a force F: is produced which urges the coil towards an-intermediate or midscale position. By increasing the number of .laminations in oneol the magnetic portions nor B andzdecrea'sing the-number, of

laminations in; the other :of themagnetic portions, .the position towards which. the ,coil is urged by. the force mayber movedeither irection and to adesired; extent away fromthe midscale position. The principles involved have beenv discussed above witlrreierence to Fig. 5a.

For complete compensation of the forceF, the magnetic cores Al and BI may-be tapered in opposite directions in conformance with-the principles discussed with referencetoFigsr- 1; 2 and:3 or. with reference to-Figs. l2oand 1.3. As still further examples, theauxiliary mechanisms .of' Figs. 6 to 11 may be, employed in accordance with the discussions thereof. Thacompensation illustrated'in Fig. 14 is based, on thatshownin Figs. 6 and 7.

The-magneticv secticn-A has secured thereto a magnetic strip late-having ends 1580; andi521; bentto form pockets-lathe mannerdiscussed with reference to the strip 2501 Figs. 6 and 7. Except for the increased range of angular movement of the-coil C3, the strip I59 operates in the same manner as the, strip. I25 ofFig; 6 to coinpensate for the force F. .It will, be understood that the coil'CZiof Fig. 14 is long enough to be lifted between the ends of thestrip. ltdto clear the strip preparatory to removal of the coil from its associated magnetic structure.

Although. the invention has: been described with reference to certain specific embodiments thereof, numerousmodifications are possible-and it is desired to cover all modifications falling withinthe spiritand scope of the invention.

'1 claim as my invention:

1. In an electrical instrumenha stator structure, a-rotorassembly including a'coil, :means mounting the rotor assembly for rotating the coil relative to the stator'structure through a'predetermined path, means responsive, toenergization of said coil alone for-develmaing solenoid torques urging the coil relative to the stator, structure towards an intermediate. positionin said path, and compensating means forproducingtorques acting between the rotor assembly and thestator structure for urging the coil away from the intermediate position in a direction vdetermined-byxthe position of the coil relative to the intermediate position to compensate for said first-named torques.

2. .An electrical instrument asdefined in claim 1 wherein said coil and said last-named means are connected for energization from a common source of energy.

3. In an electrical instrument, a magnetic structure having an air gap, a rotor structure comprising an armature assembly, said armature assembly includinga coil having a coil side disposed in said airgap, means mounting therotor structure for rotation relative; to the magnetic structure to move the coil side throughthe air gap in a predetermined path, said coil when-energized developing a solenoid torque relativeto. the magnetic structure.whichurges the coil towards 3216 an intermediate positionin said path from. any otherposition insaid path, said magnetic structure including an auxiliary air gap, the armature netic structure having'an air'gap, means mountingthe coil Witha coil side in the air gap for movement relative to the magnetic structure, said magnetic structure comprising a first magnetic portion defining a path for magnetic flux produced by electrical current flowing throughsaid coil; said magnetic portion providing a first substantial solenoid force in response to current acting between saidv magnetic portion and said coil; the solenoid force increasing as said rcoil movesin thedirectionof the force, a second mag- .netic portion defining a path for magnetic flux produced by currentflowing through saidcoil, said second magnetic portion providing a second substantial magnetic solenoid force, in response to current-in said, coil acting between said second magnetic portion and said coil, the second solenold force increasingas said coil moves in the direction of the second solenoid iorcaand means mounting said magnetic portions to direct said forces inopposition, to each other, said increases in the solenoid forces in response to movement of the coilbeing proportioned to compensate for the force resulting from magnetic flux crossing and reerossing .the first-named air gap produced .by current flowing through the coil which urges the coiltowards an intermediate position in its path of .travel.

5. An instrument, as defined-in claim 4-in combination with means for establishing a magnetic field in said air gap, whereby the movement of the coil in response to reaction of current flowing therethreugh with said magnetic field is substantially independent of said forces.

6. man electrical instrument, a pair of similar magnetic structures, each of said magnetic structures comprising a hook-shaped magnetic core having a hook section and a shank section projecting from the hook section, each of said magnetic structures also comprising a magnetic member surrounding, but spaced from, a substantial portion of the associated hook section to form therewith an arcuate .air gap, said hook section,

shanksectionand magnetic member defininga magnetic path for directing magnetic flux through the associated air gap, means mounting the magnetic cores-with their, hook sections extendingarounda common axis, one of the magnetic structures being reversed relative to the other of the magnetic structures about a line perpendicular to said axis, a coil having a coil side disposed in the air gaps, means mounting the coil for rotation relative to the magnetic structure aboutsaidaxis; each of said hook sections being shaped toprovide asolenoid force in response to current flowing through the coil which varies as the coilmoves from a position adjacent the associated shank section to a position displaced therefrom,:the'oppositelydirected solenoid forces. of

the twostructures having-a: resultant proportioned to compensate forthef forcesproduced by 17 magnetic flux produced by current flowing in the coil which magnetic flux does not flow through the shank sections.

7. In an electrical instrument, a magnetic structure having an arcuate air gap, said magnetic structure comprising a plurality of laminations of magnetic material, certain of said laminations having pole faces adjacent said air gaps which diner in configuration from other of said magnetic laminations to produce a resultant air gap having desired characteristics, a second magnetic structure similar to the first-named magnetic structure but reversed relative thereto about a line perpendicular to the axis of the air gaps, a coil having a coil side disposed in said air gaps, and means mounting the coil for rotation relative to the magnetic structures.

8. In an electrical instrument, a pair of substantially similar magnetic structures having air gaps substantially concentric about an axis and aligned to produce a resultant air gap; each of said magnetic structures comprising a plurality of laminations of magnetic material certain of the laminaticns having pole faces defining a portion of the resultant air gap having a length which varies as a function of the angle about said axis; said magnetic structures being reversed relative to each other about a line transverse to said axis, a coil having a coil side disposed in said resultant air gap, and means mounting the coil for rotation relative to the magnetic structure, said variations in air gap length being proportioned to compensate forces acting on said coil as a result of current flowing through the coil.

9. An electrical instrument as defined in claim 8, certain of the laminations of each of said magnetic structures having an air gap of uniform length throughout the path of travel of said coil side.

16. An electrical instrument as defined in claim 8, said magnetic structures being spaced along said axis by a, distance sufiicient to permit rotation of a side of said coil therebetween, and means for establishing a magnetic field in said resultant air gap.

11. In an electrical instrument; a pair of substantially similar magnetic structures having air gaps substantially concentric about an axis and aligned to produce a resultant air gap; each of said magnetic structures comprising a plurality of laminations of magnetic material certain of the laminations having pole faces defining a portion of the resultant air gap having a dimension at angular positions about said axis which differs from the corresponding dimension of other of said laminations; said magnetic structures being reversed relative to each, other about a line transverse to said axis, a coil having a coil side dis-,

posed in mounting said resultant air gap, and means the coil for rotation relative to the magnetic structure, said variations in air gap dimension being proportioned to compensate forces acting on said coil as a result of current flowing through the coil.

12. In an electrical instrument; a pair of substantially similar magnetic struct' res; each of said magnetic structures comprising a plurality of magnetic laminations having two pairs of inner and outer pole pieces defining two air gaps spaced angularly about an axis; means mounting said magnetic structures reversed relative to each other about a line transverse to the axis with the air gaps of the two magnetic, structures in alignment, coil means having a separate coil ide disposed in each pair of aligned air gaps, means mounting the coil means for rotation about the axis, the inner pole pieces of the magnetic structures being spaced to provide a passage through which the coil means may be introduced into operative position and removed from the magnetic structures without disturbing the magnetic structures, each of said inner pole pieces having a configuration providing a varying solenoid force acting between the magnetic structure and the coil means in response to current flowing through the coil means which varies as said coil means moves over the last-named inner pole piece, the inner pole pieces of the magnetic structures having opposed solenoid forces proportioned to produce zero resultant force acting on the coil means in response to current flowing through the coil means for all positions of the coil means in the normal path of rotation of the coil means.

13. In an electrical instrument as defined in claim 12, means for producing a magnetic held in said air gaps, the configuration of said inner pole pieces being determined by dilierent shaping of a plurality of the laminaticns forming each of the inner pole pieces.

14. In an electrical instrument, a stator assembly, a coil, and means mounting the coil for rotation relative to the stator assembly about an axis, said stator assembly including a pair of magnetic core members projecting through said coil, said magnetic core members bein spaced axially of said axis and having uniform ends adjacent the radial sides of said coil, pole piece members cooperating with the cores to define air gaps for the coil, at least part of the members having dimensions adjacent the coil which vary in size with the angular position of said coil, the variation in dimensions being produced by variations in the members intermediate said ends to modify the torque acting between the coil and the stator when the instrument is energized, whereby the response of said instrument is substantially independent of the axial adjustment of the coil along said axis.

15. An instrument as defined in claim it wherein said magnetic cores comprise magnetic laminaticns, and the magnetic laininations adjacent the space between the cores are smaller than laminations displaced from said space to produce said variation in dimension.

16. In an electrical instrument; a stator magnetic structure including a magnetic core arcuate about an said magnetic core having an outer pole face and an inner pole face, and a pole piece spaced. from said outer pole face to define therewith an air gap; a rotor assembly including a coil having a first coil side disposed in said air gap, said coil having a second coil side intermediate said inner pole face an: the axis, means mounting the coil for rotation about said axis relative to the magnetic structure, said magnetic core when current flows through the coil alone coacting with the coil to develop solenoid forces urging the coil towards an intermediate position in its path of travel, and a U-shaped magnetic element surrounding said second coil sid and movable with the coil, said magnetic element having its tips adjacent the inner pole face to form with the magnetic core a path for magnetic flux produced by current flowing in the coil, said inner pole face being shaped to produce torques acting between the magnetic element the magnetic core to urge said magnetic element away from the intermediate position in a direction determined by the position of the coil relative to the intermediate position to compensate for the solenoid forces when the coil is displaced from the intermediate position.

17. The method of calibrating a moving coil instrument having a laminated magnetic structure providing po'le pieces defining an air gap for a side of the moving coil which comprises bending in one of the pole pieces a portion of a lamination relative to an adjacent lamination of the magnetic structure adjacent the air gap to modify the distribution of magnetic flux in the air gap.

18. In an electrical instrument, a pair of magnetic structures each having an air gap, a coil having a coil side disposed in said air gaps, means mounting the coil for movement relative to the magnetic structures, said magnetic structures establishing magnetic paths for magnetic flux produced by current flowing in the coil proportioned for developing forces in response to current flowing through the coil which, if equal, urge the coil towards an intermediate position in its path of movement, said magnetic structures being proportioned to produce unequal solenoid forces acting on the coil in response to current flowing through the coil, and means mounting the magnetic structures to direct said solenoid forces in opposition for urging said coil away from said interminate position towards a predetermined position.

19. In an electrical instrument, a stator structure, a coil, means mounting the coil for rotation relative to the stator structure, said stator structure comprising a pair of magnetic structures each having inner and outer pole pieces spaced to define an arcuate air gap for reception of a coil side of the coil, each of the magnetic structures producing a solenoid force responsive to current flowing through the coil for urging the coil towards an end of the path of travel of the coil, means mounting the magnetic structures reversely relative to each other for directing said solenoid forces in opposite directions, said magnetic structures providing unequal magnetic paths for magnetic flux produced by current flowing in the coil to make said oppositely directed solenoid forces unequal in magnitude, whereby the resultant of said solenoid forces urge said coil towards a predetermined position.

20. An instrument as defined in claim 19 in combination with winding means effective when energized for establishing magnetic fields in the air gaps, said magnetic structures being constructed of similar magnetic laminations, one of said magnetic structures having a larger number of said laminations than the number employed for the other of the magnetic structure.

21. An electrodynamic instrument comprising a winding, a soft magnetic structure having an air gap through which is directed magnetic flux produced by current flowing in the winding, a coil, means mounting the coil with a portion of the coil in the air gap for movement therethrough in accordance with the reaction between current flowing in the coil and magnetic flux in the air gap produced by current flowing in the winding, said magnetic structure being responsive to energization of the coil alone to produce a solenoid force urging the coil towards a predetermined intermediate position in the path of travel of the coil, and compensating means energized in accordance with the current flowing in the coil for urging the coil away from the intermediate position in a direction dependent on the position of the coil relative to the intermediate position to compensate for the solenoid force.

'22. An electrodynamic instrument as defined in claim 21 wherein the compensating means provides an air gap configuration for said coil which varies for different positions of the coil portion therein.

23. An electrodynamic instrument as defined in claim 21 wherein the magnetic structure comprises a pair of hook-shaped magnetic cores passing in opposite'directions through the coil, and pole piece means spaced from the magnetic cores to define an arcuate air gap within which said portion of the coil is mounted for rotation.

24. An electrodynamic instrument as defined in claim 23 wherein the compensating means includes configurations of the magnetic structure by which the air gaps between the magnetic cores and the pole piece means vary in opposite directions as the angular position of the coil portion in the air gaps changes.

25. In an electrical instrument; a pair of substantially similar magnetic structures; each of said magnetic structures comprising two pairs of inner and outer pole pieces defining two air gaps spaced angulariy about an axis; means mounting said magnetic structures reversed relative to each other about a line transverse to the axis with the air gaps of the two magnetic structures in alignment, coil means having a separate coil side disposed in each pair of aligned air gaps, means mounting the coil means for rotation about the axis, the inner pole pieces of the magnetic structures being spaced to provide a passage through which the coil means may be introduced into operative position and removed from the magnetic structures without disturbing the magnetic structures, each of said inner pole pieces having a configuration providing a varying solenoid force acting between the magnetic structure and the coil means in response to current flowing through the coil means which varies as said coil means moves over the last-named inner pole piece, the inner pole pieces of the magnetic structures having opposed solenoid forces proportioned to produce zero resultant force acting on the coil means in response to current flowing through the coil means for all positions of the coil means in the normal path of rotation of the coil means.

26. In an electrical instrument, a stator assembly, a coil, means mounting the coil for rotation about an axis relative to the stator assembly, said stator assembly including a magnetic core unit positioned within the coil and a magnetic polepiece unit spaced from the magnetic core unit to define an air gap for a portion of the coil, one of the units having dimensions which vary with the angular position of the coil relative thereto for the purpose of varying the response of the instrument to various energizations thereof, said variations in dimensions being located intermediate the axial ends of the last-named unit, said axial ends being substantially uniform adjacent the path of travel of the coil for all angular positions of the coil to make the response of the instrument substantially independent of the axial adjustment of the coil along the axis.

27. In an electrical instrument, a coil, a magnetic structure having an air gap, means mounting the coil with a coil side in the air gap for movement relative to the magnetic structure, said magnetic structure defining a magnetic path for magnetic flux produced by current flowing in the coil which produces a solenoid force acting between the coil and the magnetic structure, compensating means energized in accordance with current flowing in the coil for producing a second force acting in opposition to said solenoid force between the magnetic structure and the coil and proportioned to compensate fully for the solenoid force in all positions of the coil side in its path of movement relative to the magnetic structure, said compensating means comprising a second magnetic structure having an air gap for receiving said coi1 side, said second magnetic structure defining a magnetic path for magnetic flux produced by current flowing in the coil which produces a solenoid force acting between the coil and the magnetic structure in opposition to the first-named solenoid force, and means associated with the magnetic structures for directing magnetic flux through said air gaps to establish magnetic fields in the air gaps, said air gaps tapering in opposite directions corresponding to the two directions of movement of the coil relative to the magnetic structures by amounts proportioned to make the two solenoid forces equal in magnitude for all operating positions of the coil in the air gaps.

28. In an electrical instrument, a magnetic structure having an air gap, a coil having a side disposed in said air gap, means mounting said coi1 for rotation relative to said magnetic structure, said magnetic structure and the coil developing solenoid torques which urge said coil towards an intermediate position in its path of travel, and means compensating said torque, said last-named means comprising an auxiliary magnetic structure establishing a path for magnetic flux produced by current flowing in the coil, said auxiliary magnetic structure developing torques which act in opposition to said first-named torques, said auxiliary magnetic structure comprising a magnetic element extending through the coil, the magnetic element having a separate pocket at each end of the path of travel of a coil side to provide an auxiliary air gap for receiving said last-named coil side on each side of said intermediate position, whereby said magnetic element cooperates with the coil when current flows through the coil, and when said coil is displaced from the intermediate position, to urge the coil away from said intermediate position.

DOUGLASS A. YOUNG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,098,871 Zander June 2, 1914 1,597,327 Obermair Aug. 24, 1926 2,245,781 Hickok June 17, 1941 2,438,027 Young et a1 Mar. 16, 1948 

